Yes, FLASHBACKS is a Scrabble word!

Letter values in Scrabble US: F 4, L 1, A 1, S 1, H 4, B 3, A 1, C 3, K 5, S 1.

Is it a valid word?

FLASHBACKS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a transition (in literary or theatrical works or films) to an earlier event or scene that interrupts the normal chronological development of the story
an unexpected but vivid recurrence of a past experience (especially a recurrence of the effects of an hallucinogenic drug taken much earlier)